This fitting is used if you want to put a 3/4" GHT plug on a 3/4" NPT male thread. We would use something like this for our stainless bucket filler hose which has 3/4" NPT threads. This also works great for using that same hose for an inlet hose into a pressure washer.
Here would be your Flow Path if you're doing a stainless inlet hose setup:
Hose Bib > 3/4" Female GHT Coupler > 3/4" Male GHT Plug > 3/4" Female GHT to 3/4" Female NPT (THIS PART) > T316 Stainless Bucket Filler Hose > T-304 Stainless Elbow > Brass 3/4" Male NPT to 3/4" Male GHT > 3/4" Female GHT Coupler > 3/4" Male GHT Plug > Pressure Washer Inlet